Skin and its subcutaneous tissue can greatly be expanded in area if the expansion is accomplished gradually. The extension of the skin over the pregnant female's abdomen is one example. Further, huge benign tumors often exist in various parts of the body, and the skin and subcutaneous tissue covering these tumors is greatly enlarged, but still is normal in color and texture.
A tissue expander is a device designed to be implanted beneath the skin then inflated to stretch the overlying skin and subcutaneous tissue. One use of tissue expanders is to generate an increased surface area of skin to be used for grafting or reconstruction. Another use is to slowly expand the overlying skin to create a pocket beneath the skin and subcutaneous tissue to receive a permanent prosthesis such as a mammary implant.
Silicone elastomers have long been used for the manufacture of implantable devices because they are chemically stable within the body as well as non-toxic. Notwithstanding the relative inertness of silicone elastomers, they may still provoke a foreign body reaction in some patients. When a foreign substance enters human tissue, the immediate and natural reaction of the tissues surrounding the foreign substance is to render it harmless to the rest of the body. A large, inert foreign body is encapsulated in a sheath of fibrous tissue to isolate it from surrounding tissues. Encapsulation is a defensive mechanism that occurs through a process similar to the formation of scar tissue in the healing of a wound or surgical incision. A fibrous tissue capsule will form around and completely enclose an implanted device such as a tissue expander or permanent prosthesis in an intimate fashion, conforming to the respective shapes and curvatures of the device.
Capsule formation is not a problem for the patient unless the capsule begins to contract. Contracture of the capsule around an implant causes the implant to be compressed tightly and feel very hard and rigid. Ultimately, the contracted capsule may assume a nearly spherical shape causing discomfort and constitutes a serious medical problem.
One way to remedy capsular contracture is to surgically remove the contracted capsule and implant and then insert either the same or another implant, a procedure called surgical capsulotomy or capsulectomy. Alternatively, some doctors use closed capsulotomy, a method wherein force is applied to break the capsule in situ. Of course, capsular contracture can still recur.
The genesis of capsular contracture is complex and the reasons why it occurs are not yet fully understood. Nonetheless, several different approaches to avoiding capsular contraction have been investigated. One of the most popular approaches involves the use of steroids. Steroids are known to possess anti-inflammatory and anti-fibrinogenic properties. However, the use of steroids can result in complications such as tissue atrophy and discoloration of the skin. Accordingly, controversy surrounds the use of steroids and their relative utility in preventing capsular contracture. Other drugs and techniques have also been suggested, but their utility has not yet been established.
Other approaches to the problem of capsular contracture have focused on the design of the implant. One such device (U.S. Pat. Nos. 3,366,975 and 3,559,214) is described as a permanent prosthesis made of a flexible, thin-walled container or sac composed of a material impervious to the ingrowth of fibrous tissue, such as a silicone elastomer, to the external surface of which a thin layer of a porous or open-celled material has been adheringly applied. The interior of the sac is filled with an inert material approximating the resiliency of normal mammary tissue, such as a saline solution or a silicone gel.
The porous or open-celled layer is normally composed of a polyether, polyester or polyurethane foam material. Thin layers of this type of material have been applied to the back sides of mammary prostheses so that fibrous tissue could grow into the material and thereby anchor a prosthesis securely to the chest wall (U.S. Pat. No. 3,293,663). However, case studies conducted on mammary prostheses almost completely covered with a thin foam layer indicated that the incidence of capsular contracture was reduced by the use of such prostheses (Pennisi, Polyurethane-Covered Silicone Gel Mammary Prosthesis for Successful Breast Reconstruction, Aesthetic Plastic Surgery, Vol. 9 at 73 (1985); Ashley, Further Studies on the Natural-Y Breast Prosthesis, Plastic and Reconstructive Surgery, Vol. 45, No. 5 at 421 (May 1970)). Although the cause for the reduced incidence is not fully understood, it is believed that the growth of the fibrous tissue into the open-cell layer from many directions prevents the fibrous tissue from contracting in a concerted manner. In other words, the contractions occur in many directions and tend to neutralize each other (Pennisi, supra, at 73).
However, possible problems exist with the use of polyether, polyester or polyurethane foam materials in implants. These materials apparently degrade in the body over a period of time (Brown, Lowry and Smith, The Kinetics of Hydrolytic Aging of Polyester Urethane Elastomers, National Bureau of Standards (July 1979); Sherman and Lyons, The Biological Fate of Implanted Rigid Polyurethane Foam, Journal of Surgical Research, Vol. 9, No. 3 at 167 (March 1969)). Therefore, the effectiveness of these materials for preventing capsular contracture may disappear as they degrade. These materials have also been suspected of creating problems with infection and of being carcinogenic.
To avoid the potential problems with existing foam materials and still take advantage of the reduced incidence of capsular contracture attendant with the use of prostheses having a porous or open-celled outer layer, ways have been sought to make a layer of silicone elastomer having an open-cell texture. In U.S. Pat. No. 3,852,832, a mammary prosthesis is disclosed having a fixation means attached to its back side with perforations passing therethrough and ribs projecting therefrom. This fixation means is preferably to be made of a silicone elastomer. Although no method for making such a fixation means is disclosed, it is believed that it would be separately molded. Therefore, the pattern of perforations and ribs would have to be such as to allow removal from a mold. The fixation means must then be attached to the prosthesis.
Surprisingly, tissue expanders designed for temporary implantation have not been textured. We say "surprisingly" because although a tissue expander is a temporary implant, the surface that it presents to the surrounding tissue will set the stage for capsule genesis. Tissue expanders are used to create a pocket beneath the skin of the desired size and shape to accommodate a permanently implantable prosthesis. For example Brauman (U.S. Pat. No. 4,648,880) teaches a method of texturing the surface of an implantable prosthetic device using a dacron laminate as an outer cover. A tissue expander, however, is not a prosthetic device. It is a temporarily implantable tool used to create a pocket for an implantable prosthesis. Fibroblasts, migrating to the surface of a tissue expander, will secrete fibrin. The fibrin molecules will orient themselves to encase the implant. If the surface is "rough", that is, the surface has pits or irregularities that are on the order of 10 or 20 molecular dimensions, the fibrin molecules will follow the contour of the roughened surface structure when the capsule is in its embryonic stage of development. The template thus formed will serve as a support for further deposition of fibrin, which in turn, will form a dimpled sheath around the implant. If the diameter of the pit in the surface of the device is too small, the fibril molecules will not follow the contour change during deposition. If the pits in the surface are too deep, therefor ingrowth of fibroplasts or filaments of fibrin would make it difficult to remove the expander without destroying the embryonic capsule that has been so carefully prepared to prevent subsequent contracture when the permanent prosthetic device is implanted. Accordingly, a need exists for a silicone elastomer tissue expander device having a textured, preferably open-celled, surface therefor, which will disperse or disorganize the force of encapsulating tissue to minimize formation of a spherical capsular contracture. Another object of the present invention is to provide a tissue expander which can readily be removed in its entirety, if desired, from the human body without encountering any untoward affects.